The present invention relates generally to vehicular braking systems and more particularly to vehicle braking systems having anti-skid or antilock features.
Many known anti-skid devices simulate a driver induced anti-skid technique by cyclically increasing and decreasing the braking force exerted on the wheels so that a slipping wheel having a tendency to lock is permitted to re-accelerate back to a speed corresponding to the speed of the vehicle. This is typically achieved by control valves alternately allowing fluid to flow out of and then into the brake cylinder, thus lowering and then raising the brake pressure. With such a conventional antilock braking system, the controlled wheel occasionally begins slipping too much and operates for short times with comparatively large amounts of slip. This means a level of slip high enough to reduce lateral force available for steering and vehicle stability.
While such cycling may cause a momentary reduction in braking effectiveness as well as reduced stability and steerability, and other undesirable effects, it is useful in allowing re-setting of the calculated vehicle velocity. Many antilock braking systems are invoked when a calculated wheel speed differs sufficiently from a sensed wheel speed. In a typical system, wheel speeds are used to determine slip by comparison to a computed vehicle velocity either directly or indirectly. Without a constant re-checking of the computed vehicle velocity, errors will accumulate and cause serious degradation leading to substantially reduced braking and/or reduced lateral force.
It is desirable to provide an anti-skid braking system which acts as a continuous process with the corrective action taken being proportional to the deviation from a desired performance. Such a process is relatively easy to control using conventional methods such as Proportional-Integral-Differential (PID) controllers. With such a system, a differential correction is readily included allowing the system to anticipate future conditions by reacting to the rate of change of the error condition. Also, an integral term can reduce the steady state error.
One known system for producing maximum retarding force on a continuous basis without the accumulated error problem utilizes an accelerometer to generate a single speed reference for controlling an antilock system. This system is disclosed in copending U.S. Ser. No. 08/353,861, filed Dec. 12, 1994 (assignee docket no. 223-89-0020) and assigned to the present assignee. This system maximizes road friction. On gravel roads, maximum road friction is attained with the wheels locked. Locked wheels result in a loss of stability and steerability. This system generates a single wheel reference speed for all the wheels. Differences due to cornering or differing wheel diameters are not taken into account.